The generally accepted method for compensating a lagging power factor reactance of an inductive load is by capacitors which provide a leading power factor.
In order to achieve an ideal compensation, the magnitude of the compensative leading reactive power should be made equal to the magnitude of the existing lagging reactive power, otherwise the system becomes undercompensated or overcompensated. In order to achieve an acceptable degree of closeness to the ideal compensation, it has been proposed, in the prior art, to subdivide the required total shunting capacity into a plurality "n" of capacitors, which are paralleled 1, 2, 3, . . . n by switching the capacitors into and out of circuit. The mentioned capacitor switching technique dates from before 1918; using mainly from six to nine parallel capacitors, switched by contactors whose coils are energized and de-energized by an automatic control circuitry activated by the sensing of either the power factor or the ratio of the leading to lagging reactive current or reactive power.
One great disadvantage of the capacitor switching technique is that it compelled one to compromise at a degree of compensation which deviates undesirably from the ideal value, or even from an acceptable value. When using six capacitors, for example, the deviation would amount to 1/12 of the compensable value.
In the prior art there existed fear of the so called over-improvement, a name given to a power factor correction better than 0.9. With the combination of increments in sizable capacitor steps and abrupt full current switching, over-improvement could cause over-voltages of as much as 40% on the affected power lines and subsequent damage to motor driven equiptment under certain conditions of operation.
Another disadvantage is in the capacitor switching itself. With every capacitor being switched, a surge, a spike or the like results, causing a perceivable flicker which is extremely detrimental to sensitive critical electronic loads connected to the line. In fact, capacitor switching is a primary cause of trouble in computers receiving power from lines. True, in the capacitor switching systems today, switching is made at zero voltage crossing, but this feature achieves little, because in capacitors when the voltage curve is at zero, the current curve is at the peak and vice versa.
Even in absence of sensitive electronic loads, the prior art which uses capacitor switching, which is a rough and violent physical action, frequently results in capacitor fuse blowing. This causes watt losses, kilowatt demand increases, operational expenses during any interim until those fuses are replaced.
Another trouble occurs with the prior art when the load on the power lines consists of rectifiers, and particularly of rectifiers with forced commutation, which as it is known, generate a very substantial reactive power component and also very substantial 5th, 7th and 11th harmonics. Danger exists that at a certain switching stage with the rectifier transformer winding a resonant circuit whose frequency closely coincides with the frequency of the 5th or other odd harmonic, making the current increase dangerously uncontrollable.
Another disadvantage of the prior art was that the capacitors had to be furnished with discharging resistances. When there was a power failure, or power disconnection, or a fuse blowing--and fuses blew frequently, the circuit had no path of discharge through anywhere, and without the discharge resistors, the capacitors would explode at the next energization, when added to the existing charge. These discharge resistors have the disadvantage of having certain cost, but more importantly, they consume energy incessantly.
Another disadvantage of the prior art consists in the inability of recession of the bottom portion of the reactive power correction range. When the magnitude of a corrective reactive power is being studied, the upper limit which is the highest leading reactive power, is exactly determined. As to the lowest reactive power ever needed, seldom values lower than 40% of highest reactive power are necessary. Yet in the prior art the bottom of any corrective reactive power range is always zero, which in addition to being unnecessary is inconvenient.
Another disadvantage of the prior art consists in the fact that the capacitors work at all the times at their full rated voltage, and at certain switching instances the peak voltage is enhanced beyond the rated peak voltage of the capacitors which can destroy or reduce the life time of these circuit components.
This invention increases and decreases the compensatory reactive power of the electrical system instead of a process of adding and/or subtracting capacitors, by raising and/or lowering the voltage applied to a capacitor or capacitors. It may be seen that the compensating reactive power change is proportional to the square of the voltage applied to the capacitors; by comparison, the prior art uses capacitor size change, whose effect is proportional to only the first power of the capacitor size.